Method for preventing desensitization and radio interference of radio receivers

ABSTRACT

A signal which results from cross modulation of two or more nearby received waves contained in the output of a high-frequency amplifier or mixer of a radio receiver, for example, a low-frequency signal corresponding to a channel separation frequency is extracted. The gain of the high-frequency amplifier or attenuation of a variable attenuator provided in the preceding stage of the amplifier is controlled in accordance with the level of the low-frequency signal. Alternatively, the gain of the high-frequency amplifier or attenuation of the variable attenuator is controlled in accordance with the level of the low-frequency signal when it is equal to or larger than a predetermined value. This permits normal reception of a desired wave, even if the signal receiving level is high.

TECHNICAL FIELD

The present invention relates to a method for preventing desensitizationor radio interference of radio receivers and, more particularly, to amethod for preventing a decrease in the receiving sensitivity byhigh-level reception of both of a desired wave and a near-by disturbingwave, or for preventing interference with the receiving channel by amixed component of a plurality of near-by disturbing waves withoutcontaining a desired wave.

BACKGROUND ART

Important functions of the radio receiver are such as near-by channelselectivity for eliminating near-by channels to demodulate only adesired wave, a function of preventing the radio receiver from operatingas if a desired wave is received as a result of the generation of thesame frequency component as that of desired wave by mixing of aplurality of disturbing waves, and a function of preventing interferenceby receiving channels of the same frequency with each other.

These functions are influenced by characteristics of various filters ofa receiving high-frequency circuit and an intermediate-frequencyamplifying circuit and the nonlinearity of respective circuits, and thedegree of interference or disturbance differs with the correlation ofreceiving frequencies and the received signal level.

To make these functions satisfactory for practical use, efforts havebeen concentrated on the improvement of characteristics at the devicelevel, such as enhancement of the linearity of the receivinghigh-frequency circuit and the intermediate-frequency amplifying circuitor maximization of the steepness of the filter characteristic at eachstage.

Radio receivers, however, even if made sure to have such requiredfunctions through conventional measurement, are not always capable ofgood reception under the recent radio wave environment.

According to a prior art method for measuring the near-by channelselectivity of the radio receiver, the signal level of a near-by channelis raised in a state in which a signal is received at a level higherthan a desired wave receiving sensitivity level of the receiving channel(that is, a state in which a desired wave whose SN ratio is, forexample, 6 dB higher than a desired wave signal of a 20 dB is applied tothe receiver) and a difference between the signal level of the nearbychannel and the desired wave receiving level of the receiving channel isobtained when the above-said SN ratio drops by a predetermined value,and the value thus obtained is regarded as an indication of the near-bychannel selectivity of the receiver.

Similar measurements may sometimes be made wherein two nearby waves ortwo waves bearing a predetermined frequency relationship are applied, asdisturbing waves, to the receiver at the same level.

At any rate, conventional measurements of receiver functions all utilizethe desired wave signal of the sensitivity level at the basis ofmeasurement, as typified by the above method; hence, the absolute valueof the level of the disturbing wave is inevitably very low in theevaluation of the functions.

However, there are cases where radio receivers, even if found by theabove-mentioned measurement to possess the prescribed functions, areincapable of normal reception of signals of extremely high levels, evenif the level difference between the desired wave and the disturbing waveis less than a prescribed value.

For example, in the vicinity of a base station where a multifrequencytransmitting antenna is provided, the desired wave and the disturbingwave are both received at an extremely high level, but even if thedisturbing wave is slightly higher in level than the desired wave, thatis, even if the level difference between them is only 10 dB or so, thelevel of the desired wave in the receiver is suppressed owing to thenonlinearity of the receiving high-frequency circuit--this markedlydecreases the receiving sensitivity, hindering the normal reception.

Under such conditions, not only the near-by channel selectivity but alsothe interference eliminating function are impaired; for example,frequency components nearby the desired wave receiving channel, whichare generated by harmonic waves of two or more disturbing waves presentanywhere in the same receiving frequency band or a mixture thereof,cause interference and hence pose the same problem as mentioned above.

Such a problem is encountered more frequently with the recent increasein the numbers of radio channels and base stations or repeater stations;namely, radio service zones are formed everywhere in the center of acity, in particular, where many radio stations are established--thisconstitutes a serious obstacle to mobile communications.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor preventing desensitization and radio interference of radio receiversto permit normal reception even if radio waves are received at highlevels.

To attain the above objective, according to the present invention, asignal, which results from cross modulation of two or more near-byreceived waves contained in the high-frequency amplifier output or mixeroutput of the radio receiver, for example, a low-frequency signalcorresponding to a channel separation frequency, is extracted, and thegain of the high-frequency amplifier or attenuation of a variableattenuator provided in the preceding stage of the high-frequencyamplifier is controlled in accordance with the level of the extractedsignal when it is higher than a predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of a radio receiverembodying the present invention;

FIGS. 2a, b and c are spectrum diagrams for explaining the operation ofthe embodiment of the present invention;

FIG. 3 is circuit diagram showing an example of a receivinghigh-frequency amplifier for use in the present invention;

FIG. 4 is a circuit diagram showing an example of cross-modulated signalextracting means; and

FIG. 5 is a graph showing a desensitization characteristic of thereceiver according to the present invention, for evaluation of itsperformance, together with similar characteristics of conventionalreceivers.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will hereinafter be described in detail withreference to the accompanying drawings.

FIG. 1 illustrates in block form an example of an FM receiver embodyingthe present invention.

In FIG. 1, reference numeral 1 indicates an antenna, which is connectedto a receiving circuit which is made up of a cascade connection of avariable gain type high-frequency amplifier 2, a high-frequency filter3, a mixer 4, an intermediate-frequency filter 5, anintermediate-frequency amplifier 6, a demodulator 7 and a low-frequencyamplifier 8 and a local oscillator 9 connected to the mixer 4. In thisembodiment the output of the mixer 4 is applied to an interferencedetector 13 which includes a low-pass filter 10 having a cut-offfrequency of, for example 50 kHz or so, a cross-modulated signalamplifier 11 and a rectifier 12, and a DC signal obtained with therectifier 12 is fed back to a gain control terminal of theabove-mentioned variable gain high-frequency amplifier 2.

With such a circuit arrangement, the operation of the receiver portioncomposed of the antenna 1, the high-frequency amplifier 2, the band-passfilter 3, the mixer 4, the intermediate-frequency filter 5, theintermediate-frequency amplifier 6, the demodulator 7, the low-frequencyamplifier 8 and the local oscillator 9 is the same as in the prior art,and hence no detailed description will be given of the operation, butthe receiver of this embodiment differs from the conventional receiversin that the gain of the high-frequency amplifier 2 is variable with theabove-noted DC signal.

Further, the interference detector 13 in this example detects thepresence of disturbing waves of high levels and controls the gain of thehigh-frequency amplifier 2 as described below.

FIGS. 2a, b and c are spectrum diagrams for explaining the operation ofthe interference detector 13.

Now, let it be assumed that two disturbing waves f_(U1) and f_(U2)spaced, for example, 25 kHz (which is a channel separation) apart arereceived at a higher level than a desired wave f_(D) in the samereceiving band, as shown in FIG. 2a. In the prior art harmonic waves aregenerated owing to the nonlinearity of the receiving high-frequencyamplifier 2 or mixer 4 in such an instance, and as shown in FIG. 2b, thelevel of the desired wave f_(D) is drastically reduced, while at thesame time disturbing signal components f_(Un) are generated in or nearthe desired wave receiving channel--this leads to interference orserious deterioration of the SN ratio, hampering normal reception of thedesired wave f_(D).

In this case, the output of the mixer 4 contains the differencefrequency 25 kHz between the two disturbing waves and a signal componentequal to an integral multiple of the difference frequency as depicted inFIG. 2c.

In this embodiment the above-mentioned phenomenon is noted. Alow-frequency signal contained in the output of the mixer 4, i.e. aportion of a cross-modulated component, is extracted by the low-passfilter 10 which has a cut-off frequency of, for example, 50 kHz, and itis amplified by the low-frequency amplifier 11 to a required level,thereafter being converted by the rectifier 12 to a DC signal. The gainof the high-frequency amplifier 2 is controlled in accordance with thelevel of the DC signal. According to the method described above, sinceno frequency component is extracted by the low-pass filter 10 in casetwo or more disturbing waves do not exist in the receiving band, thegain of the high-frequency amplifier 2 is not reduced at all but becomesmaximum. On the other hand, when a disturbing wave is present asmentioned above, the gain of the high-frequency amplifier 2 is decreasedin accordance with the level of the disturbing wave.. Consequently, thedegree of distortion in the amplifier 2 substantially lowers and thedegree of reduction of the level of the desired wave also decreases orthe generation of a disturbing signal component which gets mixed intothe intermediate-frequency band is prevented, thus, normal reception ofthe desired wave is possible.

In other words, the method described above is an attenuation method inwhich a variable attenuator is provided between the antenna 1 and thehigh-frequency amplifier 2, and the variable attenuator reduces thesignal to the high-frequency amplifier when a level of two or moredisturbing waves are very high. In the present invention the operationtherefor is automatically performed by extracting the low-frequencysignal that results from two or more disturbing waves.

No simple-structured means has been proposed so far for directlyextracting the two or more high-frequency disturbing wave which causecross modulation in the desired wave receiving frequency band, andconventionally it would be necessary therefor to employ a complicatedand expensive device which is provided with receivers corresponding todisturbing wave frequencies and rectify amplified intermediatedfrequencies, just like what is called a frequency selective amplifier.

In contrast thereto, according to the above-described method of thepresent invention, the presence of disturbing waves can be detected withmuch ease by extracting a signal which results from inevitable mixing oftwo or more disturbing waves and the desired wave. In addition, theabove signal is a low-frequency signal of about the afore-mentionedchannel separation frequency, and hence is easy of selection andamplification. Hence, the method of the present invention can beimplemented at low cost.

FIG. 3 is a circuit diagram illustrating principal parts of a specificoperative example of the variable gain type high-frequency amplifier 2.

In FIG. 3, reference character TR₁ denotes a common-emitter typehigh-frequency amplifying transistor, which is supplied at its collectorwith power via a primary coil of a tuning transformer T₁. A bias currentsupply resistor R₁ is connected between the base of the transistor TR₁and a power supply line and the collector of a second transistor TR₁ isconnected between the base and the ground via a choke coil L. Thetransistor TR₁ has its emitter grounded. A received signal from theantenna is applied via a DC blocking capacitor C₁ to the base of thetransistor TR₁.

A capacitor C₂ and a resistor R₂ are connected in parallel between thebase of the second transistor TR₂ and the ground, and the base of thetransistor TR₂ is connected to the collector of a third transistor TR₃via rectifying diodes D₁ and D₂ and a capacitor C₃.

The collector and the base of the third transistor TR₃ are supplied withthe power source voltage via resistors R₃ and R₄, respectively, and across-modulated signal based on the above-said disturbing waves, derivedby the low-pass filter 10 from the mixer 4, is applied via a capacitorC₄ to the base of the transistor TR₃. The transistor TR₃ is used as alow-frequency amplifier for amplifying the above-mentionedcross-modulated signal.

With such a circuit arrangement, the collector-emitter resistance of thetransistor TR₂ varies with the level of the signal input to the base ofthe transistor TR₃, accordingly the base bias of the transistor TR₁ ofthe high-frequency amplifier 2 varies, permitting control of its gain.In this sense the second transistor TR₂ serves as a grounding side biasresistor of the transistor TR₁.

The above-mentioned choke coil L is to block a high-frequency signal butneed not always be provided when the collector-emitter impedance of thesecond transistor TR₂ is high.

With this method, it is considered that a decrease in the base currentof the high-frequency amplifier 2 causes a decrease of its dynamicrange, resulting in increased signal distortion. It is thereforeeffective in increasing or decreasing only the high-frequency signalwithout changing the base bias of the amplifier 2 to insert a capacitorbetween the collector of the transistor TR₂ and the base of thetransistor TR₁ for isolating them DC-wise.

With such an arrangement, it is possible to attenuate only the inputsignal to the transistor TR₁ without changing its base bias and henceobviate the above-mentioned defect.

FIG. 4 illustrates an example of the circuit connection for extractingthe afore-mentioned mixed frequency signal from the mixer output, thatis, for extracting from the normal intermediate-frequency signalcomponent a low-frequency signal component resulting from distortion ofthe disturbing waves. Conventionally, in many cases a decouplingresistor R_(D) is connected in series to the primary coil of a tuningtransformer T₂ of the mixer output portion and a by-pass capacitor C_(P)for grounding the hot side of the resistor R_(D) high-frequency-wise isalso connected to the primary coil, and only the low-frequency signalvoltage by the disturbing waves appears in the mixer output portion.Accordingly, such a connection as shown in FIG. 4 allows ease inextracting the cross-modulated signal for use in the present invention.

In this instance, the afore-mentioned low-pass filter 10 need not alwaysbe provided if the intermediate-frequency signal, the receivedhigh-frequency signal, a local oscillation signal and their mixed outputare sufficiently removed by this method to extract only thelow-frequency signal based on the disturbing waves.

FIG. 5 is a graph showing measured results of the performance of areceiver embodying the present invention. For better understanding ofthe effect of the invention, results of performance tests ofconventional receivers are also shown.

In the performance test the desired wave f_(D) of 400.56875 MHz and fourdisturbing wave signals f_(U1) through f_(U4) which are 493.75 kHz lowerthan the desired wave f_(D) (the disturbing wave signals beingsequentially spaced 25 kHz apart) are simultaneously applied to thehigh-frequency input end of the receiver via a required couplingcircuit. In FIG. 5, the abscissa represents the disturbing wave level(all the levels of the disturbing wave signals being the same) and theordinate represents the level of the desired wave signal correspondingto a predetermined SN ratio (20 dB, for instance) reached when therespective disturbing wave signals were input.

In the measurements the frequency of the desired wave signal was400.56875 MHz as mentioned above and frequencies of the four disturbingwave signals were 400.000, 400.025, 400.050 and 400.075 MHz, and threeconventional receivers fabricated on the basis of different designs wereemployed.

As is evident from FIG. 5, in the conventional receivers the desiredwave receiving sensitivity stays at a predetermined value of about -5dBμ until the disturbing wave level reaches approximately 60 dBμ, but asthe disturbing wave level further increases, the desired wave receivingsensitivity rapidly deteriorates, and when the disturbing wave level is100 dBμ, the desired wave signal cannot normally be received unless itslevel is in the range of 65 to 80 dBμ.

This means that the desired wave signal cannot be received or the SNratio is seriously impaired when the level of the disturbing wave signalgoes very high, for example, when the level is only 20 to 35 dB or sohigher than the level of the desired wave.

On the other hand, the receiver according to the present invention isable to receive the desired wave at a level of 45 dBμ even when thelevel of the disturbing wave is as high as 100 dBμ, and the level of thedesired wave receivable by the receiver is about 10 dBμ when thedisturbing wave is at an intermediate level, for instance, 80 dBμ,whereas in the conventional receivers the level of the desired waveneeds to be in the range of between 35 and 45 dBμ in such a case. Thedifference of 25 to 35 dBμ is the difference in sensitivity between thereceiver according to the present invention and conventional receivers.This demonstrates that the effect of improvement by the presentinvention is very marked.

In the above-mentioned measurements the channel separation of thereceiving frequency band is 25 kHz and the cut-off frequency of thelow-pass filter 10 for extracting the cross-modulated signal must be setto a value equal to or higher than at least 25 kHz but lover than theintermediate frequency; however, when the channel separation is 12.5kHz, the low-pass filter 10 needs only to extract a frequency componentof at least 12.5 kHz.

Although the above embodiment has been described to derive thecross-modulated signal from the mixer 4, it is evident that theinvention is not limited specifically thereto but that the signal canalso be derived from, for example, the output of the high-frequencyamplifier or similar element.

The variable gain type high-frequency amplifier 2 need not be limitedspecifically to that exemplified in FIG. 3 but may also be modified intovarious forms.

While in the above embodiment the gain of the high-frequency amplifier 2is increased or decreased by controlling the base bias of the transistorTR₁ forming the amplifier 2, it is also possible to connect a variableattenuator between the antenna 1 and the high-frequency amplifier 2 andcontrol its attenuation. When the attenuator is excellent in linearity,the characteristic shown in FIG. 5 can be further improved.

That is, in the above embodiment the linearity of the high-frequencyamplifier 2 is likely to deteriorate, because its gain is reduced bydecreasing the base bias current of the transistor TR₁, but the use ofan attenuator of excellent linearity affords reduction of the level ofthe signal input to the high-frequency amplifier 2 without degrading itscharacteristic; hence, the desired wave receiving level selectingcharacteristic can be maintained theoretically within the attenuationvariable range of the attenuator.

For example, in the case where the desired wave receiving levelselecting characteristic is 80 dBμ, the desired wave can be receivedwhen its level is 20 dBμ, even if the level of the disturbing wave is100 dBμ.

As the variable attenuator of excellent linearity for use in the presentinvention, it is possible to employ a variable attenuator forhigh-frequency signal use which has an arrangement in which itsattenuation is controlled by electrically switching, from the outside,electronic switches connected to a plurality of terminals of aresistance network or ladder-type resistance network.

If such a variable attenuator is allowed to be somewhat nonlinear, thenthe attenuator can be implemented by voltage or current control of atransistor, FER, diode, or similar semiconductor device.

For controlling the variable gain type high-frequency amplifier orvariable attenuator a comparator or logarithmic amplifier may also beemployed in the afore-mentioned interference detecting portion, by whichthe gain of the amplifier or attenuation of the attenuator arecontrolled for only disturbing waves of levels above a certain value, orthe gain is reduced or attenuation is varied greatly as the disturbingwave level increases.

in recent years there has been studied, for a novel receiving system, apractical application of a method in which a received signal and a localoscillation signal of the same frequency are mixed together and themixed signal is used to directly demodulate a signal superimposed on acarrier. It will be appreciated that the present invention is alsoapplicable to such a method.

As described above, according to the present invention, it is possibleto prevent, by a simple method, radio interference or desensitization ofa radio receiver which occurs when two or more disturbing signal levelsare high. Thus, the present invention produces a remarkable effect inimproving the functions and performance of radio receivers.

We claim:
 1. A radio receiver capable of preventing radio interferencecaused by cross modulation of two or more disturbing waves, comprising:ahigh-frequency amplifier for amplifying a received signal, a gain ofsaid high-frequency amplifier being controlled by a direct currentsignal; a local oscillator; a mixer for outputting an intermediatefrequency signal caused by mixing an output signal of said localoscillator and an output signal of said high-frequency amplifier; andinterference detector means for detecting a cross modulated productwithin said intermediate frequency signal, comprising a filter whichseparates said cross modulated product from said intermediate frequencysignal, said cross modulated product having a low frequency which is atleast equal to the channel separation frequency of said two or moredisturbing waves, and a rectifier for generating said direct currentsignal from said cross modulated product for controlling said gain ofsaid high-frequency amplifier.
 2. A radio receiver capable of preventingradio interference caused by cross modulation of two or more disturbingwaves, comprising:an attenuator which controls an attenuation level of asignal by application of a direct current signal; a high-frequencyamplifier for amplifying an output signal of said attenuator; a localoscillator; a mixer for outputting an intermediate frequency signalgenerated by mixing an .output signal of said local oscillator and anoutput signal of said high-frequency amplifier; interference detectormeans for detecting a cross modulated product within said intermediatefrequency signal, comprising a filter which separates said crossmodulated product from said intermediate frequency signal, said crossmodulated product having a low frequency which is at least equal to thechannel separation frequency of said two or more disturbing waves, and arectifier for generating said direct current signal from said crossmodulated product for controlling said attenuation level.
 3. A radioreceiver according to claim 1, wherein:said high-frequency amplifier iscomprised of at least two transistors, a base of the first transistorreceiving a signal from an antenna of said radio receiver, said basebeing connected to a capacitor, said base of said first transistor alsobeing connected to a ground via a choke coil and a collector and anemitter of the second transistor, said choke coil being capable ofobstructing passage of high-frequency signals, a collector of said firsttransistor being connected to a power supply via a primary coil of atuning transformer, a base of said second transistor being connected toa rectifier, said rectifier being connected to said interferencedetector means to receive said direct current signal.